Limiting dietary protein intake results in amino acid deficiency within cells and activates several signal transduction pathways collectively called the amino acid response (AAR). A number of genes have been identified that are transcriptionally-activated by the AAR, including the bZIP transcription factor ATF3, for which cellular stress induces multiple isoforms by pre-mRNA alternative splicing. Two of these isoforms, full- length ATF3 (ATF3-FL) and a form with a truncated leucine zipper, ATF3?Zip3, are induced in expression by low protein diet in vivo or by amino acid deprivation of cultured cells. These two isoforms exhibit opposing action on the AAR target gene encoding asparagine synthetase (ASNS); exogenous ATF3-FL expression causes transcriptional repression of the amino acid-dependent induction of ASNS, whereas ATF3?Zip3 further enhances the induction. How the cellular amino acid content signals to and controls pre-mRNA alternative splicing has not been investigated. In fact, the study of the regulation of alternative splicing by macro-nutrients represents an entirely new area of investigation in the splicing field. The hypothesis is that ATF3 isoforms have opposing actions within the cellular response to protein/amino acid stress and that the individual isoforms interact with activity-modifying proteins and/or transcriptional co- regulators that support these opposing activities. To address this global hypothesis, three sub- hypotheses will be tested. Hypothesis I: There are differences in the synthesis and functional activities of specific ATF3 isoforms induced by dietary low protein in mice and amino acid deprivation of cultured cells. The proposed research will investigate the kinetics of synthesis for ATF3-FL and ATF3?Zip3 and the functional consequences of each isoform will be addressed by RNA and protein microarray analysis in transgenic mice expressing either ATF3-FL or ATF3?Zip3 individually. Hypothesis II: Amino acid-dependent signaling pathways regulate the alternative splicing of ATF3 during the AAR. These studies will determine the signaling pathway responsible for sensing and transducing the amino acid deficiency signal to the proteins that regulate exon choice during alternative splicing. Hypothesis III: Protein-protein interactions of individual ATF3 isoforms modulate their action on AAR target genes. ATF3-interacting proteins will be identified and their role in the AAR determined. Collectively, the proposed studies will provide novel information and address significant gaps in our knowledge of ATF3 alternative splicing and ATF3 isoform function. The insight gained from these studies will impact the fields of: 1) macro-nutrient control of pre- mRNA alternative splicing; 2) amino acid-dependent control of transcription; and 3) ATF3 function in nutrition and disease.